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156

HYDRAULIC LIMES AND CEMENTS

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a trowel and are taken from the moulds after 24 hours, when they are
immersed in water.

Greater value attaches to the results of compression tests. According
to the Italian Regulations, cubical briquettes of 50 sq. cm. face are made
in suitable moulds by compressing with i(>o blows from a 3 kilo hammer
falling 0-5 metre and developing 0-30 kilogram-metre per grain of dry sub-
stance. The briquettes are kept and the results expressed as with tensile
strength, the same being the case with briquettes of pure cement. The
compressive stress should be exerted on two opposite, faces which have been
in contact with lateral walls of the. mould.

Bending tests are also sometimes made, and for practical purposes, tests
of the indeforniabiliiy or constancy of volume in the cold and in the hot are
of importance. Further, in some, cases, it may be advisable to make tests
of adhesion, porosity and ficrmcabiitiy. For these tests and for greater
details of the compression and tension tests, reference must be made to
special treatises.

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*   >H

Hydraulic- Iwit'K have the specific, gravity ^-5- >','<> or, inoro commonly, 2.7-
2*83, and their apparent density is usually 0-5 o-H, and may be even greater with,
those having very marked hydraulic, properties. The e.hemical composition
varies somewhat, but in ordinary eases the proportion of clay is .HI .{o%. The
hydraulic index may vary between fairly wide limits, but in most, instanc.es is
less than 0-50. The loss on calcination is usually S t>",', but may reach Jo%
or more ; as regards alkalinity, the solution obtained from 11-3 gram of substance
requires about .'.o e.e. of N/io- acid. For a paste of normal consistency, hydraulic
limes almost, always require more than .|o% and sometimes even 00% of water;
setting, in either air or water, seldom takes place in less limn a day, and often
takes longer.

According to the Official Italian lit^itlatitiHx, hydraulic lime should have a
specific gravity of at least. 2-7 and should not, leave more than /''';', of residue
on a sieve of qoo holes per sq. cm. and not. more than .vj';.;', on one of .pjoo holes ;
the setting of the normal paste should not begin earlier than < hours a.iul should
not end later than .|H hours. The strength of the normal mortar briquette after
'2<S days should not be less than : tension, ;j kilos per sq. em, for ordinary hydraulic,
lime and >S for those with marked hydraulic qualities ; compression, .',;> kilos
and 50 kilos per sq, cm. respectively.

Natural crnic-nts usually have the specific gravity. ,',-S r\, ami the apparent
density 0-7 i for quick-setting, and sometimes as much a,s P.; for slow setting
cements, The hydraulic, index is usually 0-30 o-Ko or more, and is greater with,
quick-setting than with slow-setting products (with the former, the MieliaGlis
hydraulic modulus is generally i-,1, HI). The loss on ignition and the alkalinity
are almost always less than with the hydraulic, limes, At the most: ,<o 45%
of water is required for the normal paste and selling requires less than half an
hour for quick-setting and more than half an hour sometimes some hours 
for rapid-setting cements. Send slow-setting cements arc sometimes regarded
as an intermediate grade, but the transition from rapid" to slow setting cements
is too gradual to allow of the definition of the limits of sueh a class,

For Ynpid-Mlti'iii> cements the Official [taliitu h't'^altilions prescribe ; a. specific
gravity greater than 2-H ; not more than j,o% of residue on a, t;no mesh sieve ;
initial and final sets to occur between i minute and 30 minutes; briquettes of
normal paste to exhibit, after 7 days, tensile and eompressive strengths not less
than if> and TOO kilos respectively per sq. cm.

True Porlkmd wtnunt has title specific: gravity 3-05 ^25 and the apparent                                                                                                                       i** *j